Following the pyrolysis of certain carbonaceous materials, such as oil shale, tar sand, coal, and diatomaceous earth, to extract volatile components, such as oil and hydrocarbon gases, the solid which remains is referred to as "pyrolyzed carbon containing material" or in the case of oil shale--"retorted oil shale". This solid contains carbonaceous residue which may be burned to yield heat energy. The heat recovered from the combustion may be used to supply heat for the pyrolysis of fresh oil shale or other carbonaceous material in the pyrolysis process.
In the case of oil shale, the ash that remains after the combustion of the carbonaceous residue present in retorted shale is called "burned shale". In some retorting processes the burned shale serves as a "heat carrier material" to directly supply the heat for retorting of more raw oil shale. In such process schemes retorted shale is combusted in a separate vessel, and the hot, burned shale is recycled to the retorting vessel and mixed with the raw oil shale. Heat is transferred between the two solids, and the mixture reaches a uniform temperature. At this temperature the oil is released from the raw oil shale. See for example U.S. Pat. No. 4,199,432.
During retorting and combustion of oil shale to produce product vapors and heat, respectively, the physical integrity of the shale particles is changed, and a substantial amount of fine grained shale is produced which is not suitable for use as the recycle heat carrier material. Therefore, it is necessary to separate this fine material prior to recycling the burned shale.
The solids withdrawn from the retort and sent to the combustor are a mixture of freshly retorted shale and cooled recycle shale. The recycle shale contains little or no carbonaceous residue.
When burning the carbonaceous residue in retorted oil shale sufficient residence time must be provided in the combustor to assure both reasonably complete combustion and thermal equilibration between the hot burning particles and the cooler recycle particles. In processes using a liftpipe (entrained bed) combustor, a very long liftpipe is required to achieve a residence time of even a few seconds in the combustion zone.
The present invention is directed to an improved process for burning the pyrolyzed particulate carbon containing material and for separating out fine hot particles prior to recycling the remaining hot particles back into the pyrolysis vessel.